An amorphous oxide semiconductor thin film has a high carrier mobility and a large optical bandgap, and can be formed at low temperature. The amorphous oxide semiconductor thin film is therefore expected to be applied to a next-generation display requiring large size, high resolution, and high-speed drive, and to a low-heat-resistant resin substrate used in a transparent display or a flexible display.
Among such oxide semiconductor thin films, an amorphous oxide semiconductor thin film including indium (In), gallium (Ga), zinc (Zn), and oxygen (O) (hereinafter, also referred to as “In—Ga—Zn—O” or “IGZO”) is particularly preferred to be used because of its extremely high carrier mobility.
It is however known that an electron state of the oxide semiconductor thin film is varied due to a film formation step and subsequent heat treatment, affecting quality of TFT. For example, carrier concentration that dominates TFT characteristics is greatly varied due to lattice defects formed during the film formation step and hydrogen in the film, allowing the TFT characteristics to be easily varied. In a manufacturing process of a display or the like, therefore, in light of improving productivity, it is important that properties of an oxide semiconductor thin film are evaluated, and results of the evaluation are fed back to adjust a manufacturing condition for quality control of TFT.
The inventors have disclosed a method for evaluating mobility of an oxide semiconductor thin film by a noncontact method without providing an electrode in PTL 1, in which mobility of the oxide semiconductor thin film is qualitatively or quantitatively evaluated by a microwave photoconductive decay method (hereinafter, also referred to as “μ-PCD method”).
For the TFT including the oxide semiconductor thin film, it is important to control not only mobility as a basic transistor characteristic but also product defects such as unevenness in luminance, washed-out color, and bad display. To achieve this, it is also required to have good resistance (hereinafter, also referred to as “stress resistance”) against stress such as light irradiation or voltage application as a cause of such product defects. The stress resistance means that even if a semiconductor device such as a transistor receives stress, for example, even if the semiconductor device is continuously irradiated with light or continuously receives a gate voltage, the semiconductor device maintains good properties.
In one type of stress resistance, threshold voltage (Vth) does not shift in a drain current-gate voltage characteristic (hereinafter, also referred to as “I-V characteristic”), i.e., the amount of change in Vth (hereinafter, also referred to as “ΔVth”) between before and after stress application is small. For example, in an organic EL display, a positive voltage (hereinafter, also referred to as “positive bias”) is continuously applied to a gate electrode of a drive TFT during light emission of the organic EL display. Hence, Vth is varied and thus a switching characteristic is disadvantageously varied. In a liquid crystal television, TFT is irradiated with light from a backlight. Hence, if such light irradiation is continued in addition to application of a negative voltage (hereinafter, also referred to as “negative bias”), Vth is varied and thus a switching characteristic is disadvantageously varied.
In addition, a good initial repetition characteristic is necessary as stress resistance associated with the positive bias. The initial repetition characteristic means a difference between Vth calculated from an I-V characteristic obtained at first measurement and Vth calculated from an I-V characteristic obtained after multiple times of measurement when the I-V characteristic is measured multiple times after TFT is manufactured. The initial repetition characteristic is better as the difference in Vth is smaller.
With the stress resistance associated with the positive bias, it is also necessary that Vth of TFT is controlled within an appropriate range. If Vth has a minus value, current flows when the gate voltage is not applied, leading to an increase in power consumption. On the other hand, if Vth has an extremely large positive value, TFT operation requires a large voltage.
Since such a variation in switching characteristic reduces reliability of the display itself, stress resistance is desired to be improved.
In addition, a process condition causing a large difference in TFT characteristics is reported. For example, NPTL 1 discloses that when an oxide semiconductor thin film is annealed, an electron state in the annealed oxide semiconductor thin film is varied depending on types of a gate insulating film used in TFT, which resultantly greatly affects TFT characteristics. In NPTL 2, it is reported in detail that the TFT characteristics are greatly affected by a type of a protective film formed on a surface of the oxide semiconductor thin film.